Organic Evolution. 195 



and antennae are concerned. First, those which preserve 

 eyes and antennas in the original absolute and relative 

 proportion and value ; secondly, those in which, while the 

 eyes remain the same, the antennas are longer and more 

 sensitive ; thirdly, those in which, while the antennas are 

 longer and more sensitive, the eyes are reduced in size and 

 elaboration. According to the principle of economy, the 

 third class have sufficient advantage over the first and 

 second to enable them to survive and escape the elimina- 

 tion which removes those with fully developed eyes. It 

 may be so. We cannot estimate the available advantage 

 with sufficient accuracy to deny it. But we may fairly 

 suppose that, in general, it is only where the useless organ 

 in question is of relatively large size, and where nutriment 

 is deficient, that economy of growth is an important factor. 



We may here note the case of the hermit crab as one 

 which exemplifies degeneration through the reversal of 

 natural selection. This animal, as is well known, adopts 

 an empty whelk-shell or other gasteropod shell as its own. 

 The hinder part of the body which is thus thrust into the 

 shell loses its protective armour, and is quite soft. Pro- 

 fessor W T eismann seems to regard this loss of the hardened 

 cuticle as due entirely to panmixia. If what has been 

 urged above has weight, this explanation cannot be correct. 

 No amount of promiscuous interbreeding of crabs could 

 reduce the cuticle to a level indefinitely below that of any 

 of the interbreeding individuals. But it is clear that an 

 armour-sheathed "tail" would be exceedingly ill adapted 

 to thrusting into a whelk-shell. Hence there would, by 

 natural selection, be an adaptation to new needs, involving 

 not the higher development of cuticle, but the reverse. So 

 far as the cuticle is concerned, it is a case of reversed 

 selection. Whether this reversal alone will adequately 

 account for the facts is another matter. 



Mr. Herbert Spencer has made a number of observa- 

 tions and measurements of the jaws of pet dogs, which lead 

 him to conclude that there has been a reduction in size 

 and muscular power due to disuse. The creatures being 



